Analysis of flow conditions in freejet experiments for studying airfoil self-noise

A new set of mean wall pressure data has been collected on a controlled diffusion airfoil at a chord Reynolds number of 1.2 x 10(5) in a freejet anechoic wind tunnel. Comparisons of the experimental data with Reynolds averaged Navier-Stokes (RANS) simulations in free air show significant flowfield and pressure loading differences, indicating substantial jet interference effects. To analyze these effects, a systematic RANS-based computational fluid dynamics study of the experimental How conditions has been carried out, which quantifies the strong influence of the finite jet (nozzle) width on the aerodynamic loading and How characteristics. When the jet width is not sufficiently large compared to the frontal wetted area of the airfoil, the airfoil pressure distribution is found to be closer to the distribution on a cascade than that of an isolated profile. The airfoil lift is significantly reduced. Accounting for the actual wind-tunnel setup recovers the wall pressure distribution on the airfoil without further empirical angle-of-attack corrections. These jet interference effects could be responsible for the discrepancies among some earlier experimental and computational studies of airfoil self-noise. They should be accounted for in future noise computations to ensure that the experimental flow conditions are simulated accurately.